Earth-related coordinates of spatial data can be either
- projected: measured on a two-dimensional surface, which is associated with a projection from the Earth’s surface
- geocentric: x-, y-, and z-coordinates associated with the three principal directions, measured from Earth’s center
- geographic: degrees latitude and longitude, associated with a datum (ellipsoidal or even spherical model of the Earth)
Geometrical computations on projected or geocentric coordinates can be done using Euclidean geometry, where all lines are staight.
Spherical (or ellipsoidal) geometry computes distances, direction, areas etc. over the surface of the sphere (ellipsoid).